Those who have used a jet printer of the type described in Australian patent specification No. 502,523 will be aware that if it is desired to produce a pattern on a fabric using dots which are formed by droplets from a number of droplet streams, with each droplet stream issuing from a separate orifice, then accurate alignment of the droplet streams and careful positioning of the droplet generators must be achieved and maintained. The achievement of the necessary alignment of the droplet stream has always been a lengthy, tedious and difficult task and, with the prior art jet printing equipment, the correct alignment has been difficult to maintain. The need for constant re-adjustment of the jet printing equipment has been a significant factor in the labour costs associated with the operation of jet printing equipment, and is one of the reasons why fabric having designs printed on it with such equipment remains expensive, notwithstanding recent technical advances in jet printing.
One method of ensuring the correct relative positioning of the droplet streams in a jet printer is to mount each jet body so that when there is zero charge on the droplets, they fall into a small opening in a collector (also called a trap or a gutter) which has been accurately positioned relative to neighbouring collectors in the jet printer. Since it is difficult to design droplet generating heads which will always produce a stream of droplets that enter their respective collectors in the absence of a charge on the droplets, some form of adjustment mechanism for the generating heads must be provided. It is a further requirement, for the most effective jet printing, that the scan amplitude of the droplet streams is either controlled or adjustable, to enable the maximum deflection of one droplet stream to be in exact registration with the minimum deflection of the next droplet stream.
For a single jet, it is possible to design a control mechanism for the jet which enables the jet stream to be precisely aligned to a required specification. But when a plurality of droplet generators are to be mounted close to each other in an array in a jet printer, the limited space available for the array of droplet generators and the need for all the adjustments to be made independently, make such designs impractical. Simple scaling down of the size of a single jet adjuster is not appropriate because the adjuster must have a certain robustness in order to maintain mechanical stability.
Another point well known to engineers who service jet printing equipment of this type is that although it is a relatively easy matter to place an orifice mount so that the aperture for the stream of droplets is precisely positioned, the same specifications of accuracy cannot be achieved with the stream axis orientation.
An analysis of the problems discussed above has shown that since the aim error of a droplet stream may be in any direction relative to the jet body axis, then two-dimensional planar correction of the aiming point is necessary. Such adjustments are possible with ball and socket joints or with universal joint arrangements, but sufficient accuracy and stability of the joints are difficult to maintain in the small sizes required. A further disadvantage of adjustment systems using such joints is evident when the droplet generating heads must be closely spaced in a linear array. In such a case, the spacing of the droplet generating heads would have to be greater than would be necessary if tilting of the generating heads using such joints were not required. Another disadvantage of such a system is that the jet or stream of droplets from the generating head may not be correctly aligned with the axis of the charge electrode when the droplet generating head is tilted. Such misalignment errors are known to contribute to charge electrode wetting by satellite drops, and to variability in the sensitivity of deflection.